High-speed printers utilize multiple-beam semiconductor lasers that are capable of transferring print data in parallel. The printing speed of such a high-speed printer is in part limited by the laser intensity (output power) and number of parallel laser beams of the printer's multiple-beam semiconductor laser. Early laser-based high-speed printers utilized edge-emitting devices, which emit light parallel to the various layers formed during fabrication (i.e., parallel the upper surface of the device “chip”). A problem with edge-emitting devices is that the number of laser beams that can practically be included in a multiple-beam laser array is four. As such, more recent laser-based high-speed printers are turning to Vertical Cavity Surface Emitting Lasers (VCSELs), which emit light perpendicular to the boundaries between the fabrication layers (i.e., perpendicular the device's upper surface), and are thus attractive light sources for multi-beam printing because they are relatively easy to form into multi-beam arrays. However, a problem associated with the use of VCSELs in high-speed printers is that they have inherently low output power, thus requiring more time for each beam to transfer sufficient energy.
Obtaining sufficient optical throughput from each VCSEL element presents a major challenge in VCSEL-based printing. The low optical throughput inherent in VCSELs is due to the small active region associated with each VCSEL element, and it is fundamentally difficult to extract high single mode output power from these devices. Much technical work and innovation has gone into improving VCSEL light outputs. But despite this effort, current VCSEL outputs remain non-optimal for printer platforms running at print speeds of 180 ppm and higher.
Various ideas have been previously proposed to increase the optical throughput of existing high-power VCSEL arrays so they can be effectively employed in high-speed printing systems. The ideas considered include the use of active cooling, increasing the effective optical magnification by rotating the VCSEL array and using interlace scanning, and the use of microlens arrays to lower the beam divergence so more light can be coupled to the optical system.
The idea of using a microlens to reduce beam divergence to compensate for the low light output powers has been previously proposed. Most VCSELs utilizing microlenses today have the lenses fabricated on the wafer backside in a monolithically integrated fashion. FIG. 10 illustrates an exemplary architecture including microlenses fabricated on a “backside” surface of a substrate (i.e., opposite to the “top” surface on which the VCSELs are fabricated). This approach, while attractive for some applications, has significant limitations. The lens diameter and spacing are necessarily large (on the order of a few hundred microns). The thickness of the substrate is difficult to control precisely, so the z-displacement tolerance of the lens relative to the light source position is substantial. The substrate has to be transparent to the wavelength of interest. Finally, the lens fabrication steps, including substrate backside processing, have to be compatible with those of the lasers. These limitations prevent the cost-effective production of VCSEL-based printing systems.
What is needed is an apparatus and method for integrating VCSEL arrays with microlens array in a manner that is practical, cost effective and optimized.